Tools for breaking or excavating with working inserts of hard metal have been produced in configurations (e.g., see European Publication No. 122 893 which corresponds to U.S. Pat. Nos. 4,938,538 and 5,161,859, the disclosures of which are hereby incorporated by reference) which have a lower energy consumption for a given operating capability.
Although the front tip of the insert is intended to provide the cutting or breaking action in these low energy tools, the softer material of the body exposed to impact or abrasion during operation of the tool can suffer wear and damage, one result of which is to weaken the attachment of the insert. The tool then fails prematurely because the insert has been dislodged.
As regards the forms of tool illustrated in European Publication 122 893, this kind of problem is more likely to be encountered when the insert is a simple cylinder with a conical tip. Other insert forms shown in that patent publication have portions much larger than the hole into which the insert fits, so that the outer regions of these conical portions provide protection for the forward end of the body. However, these alternative hard metal inserts are more difficult to produce, because their complex shapes are not well adapted to the pressure sintering method that must be used. They also require substantially more hard metal.
In Soviet Patent 899916 it has been proposed to form the hard metal insert with a large disc-like skirt intermediate its length so that when the cylindrical rear end of the insert is placed in a fitting bore in the front of the tool body the skirt covers the front face around the bore. This requires less material than the large conical portions of the inserts shown in European Publication 122 893, but the shape cannot be formed satisfactorily by sintering, because the very high sintering pressures demanded cannot be applied evenly and the insert will have weaknesses tending to produce premature failure.
German Patent Nos. 24 42 146 and 30 05 684 show tools in which the front portion of the tool body is composed of a matrix containing hard metal. This does not overcome the problem of wear because the softer mass of steel in which the hard metal is held is exposed and although the hard metal particles may not wear they will be dislodged as the steel wears. A greater concentration of hard metal in the matrix to avoid this disadvantage would lead to increased material costs without simplifying the manufacture of the tool.
The tool disclosed in British Patent 2,004,315 has, on the outer circumference of the leading end of its body, a cylindrical ring of hard metal serving as reinforcement for the body, but the metal body between the ring and the insert is still exposed to wear. Furthermore, to the extent that the outer ring functions to protect the tool against abrasion, it is not possible to use this solution to manufacture a so-called low-energy tool because the ring will only have this low energy effect if the entire front end of the tool participates in the cutting or breaking action.
Finally, there may be mentioned examples of tools which can be considered analogous to that in British Patent 2,004,315 in that, instead of the outer ring of hard metal, there is provided a layer of hard metal extending as a complete sheath over at least the forward portion of the shank. One example is to be found in U.S. Pat. No. 4,682,987 in which the hard metal is applied as a fused coating to the shank, clearly a rather complicated and expensive solution. U.S. Pat. No. 3,627,381 shows another example in which a considerably thicker sheath of hard metal is provided, adding to the cost of the tool.